The deterioration of the operating condition for a packet communication in a wire environment is attributable to the congestion resulting from concentration of many packets in a narrow-bandwidth path which forms a communication bottleneck; usually, reduction in the bandwidth is not caused by worsening of the connecting condition of the path itself. That is, the upper limit of the overall bandwidth available on the path is fixed. Accordingly, the quality of communication between transmitting and receiving terminals could be improved by detecting the congestion on the path and performing delivery control in accordance with the congestion.
On the other hand, the deterioration of the operating condition for a packet communication in the radio environment is affected not only by the packet congestion but also by aggravation of the communication circumstance due to a change in the connecting condition of the radio path.
Generally, the bit error rate of the communication in the radio environment is higher than that in the wire environment, and in the packet communication a packet containing a bit error is often regarded as invalid (i.e., a packet loss) in its entirety. For that, in many instances, an error correcting mechanism deals with the packet loss by resending the packet being lost or sending a redundant packet from the transmitting terminal in a data link layer or lower than that layer.
With the presence of the error correcting mechanism, however, it appears from the user side that the actual packet loss probability is not so high as the bit error rate. But, when the delay time by processing in the error correcting mechanism increases, it is observed as if a decrease in the throughput or in the available bandwidth is caused. As a result, the available bandwidth itself also undergoes variations. That is, the upper limit of the overall bandwidth available on the path also varies.
If the packet delivery is continued in spite of such variations, the situation will also arise where the packet transfer in excess of the available bandwidth continues, which causes a further increase in the delay or packet loss. This leads to quality deterioration of continuous media information or the like in particular which needs to be delivered on a real-time basis.
In the radio environment the communicating conditions become deteriorated typically when a mobile terminal makes handover. As a delivery control scheme adaptable to a change in the conditions of communication by handover, there is, for example, “A Traffic Control Scheme of Live Video and Audio for PHS with a 64 kbps Bearer Service of Best Effort Type,” Technical Report of IEICE, CQ99-81 (2000-02). This scheme is to measure RTT (Round-Trip-Time) between transmitting and receiving terminals and control the packet delivery according to the measured RTT value.
In addition, it is also considered to utilize intact a scheme which performs communication control according to the congestion in the wire environment. In particular, many systems using such a scheme monitor the congestion by detecting the packet loss. But the scheme based on the packet loss detection is a postprocessing type.
Since the conventional communication system has such a configuration as described above, if the communicating conditions are badly deteriorated as in the case of handover, it is possible to cope with such a situation in the postprocessing manner, but since the bandwidth available on the radio path is undetectable, it is impossible to perform appropriate packet delivery control in accordance with the communication environment.
The present invention is intended to solve such a problem as mentioned above and has for its object to provide a communication system and a transmitting terminal which permit detection of an available bandwidth in the radio path and hence ensure an appropriate packet delivery.
Furthermore, the present invention has for its object to provide a receiving terminal which is capable of offering information necessary for a transmitting terminal to detect an available bandwidth in the radio path.